Heat-expandable microspheres which have a structure comprising a shell of thermoplastic resin and a blowing agent encapsulated therein are generally called heat-expandable microcapsules. Thermoplastic resins usually used for the purpose of making such heat-expandable microspheres include vinylidene chloride copolymers, acrylonitrile copolymers, and acrylic copolymers, and blowing agents mostly employed are hydrocarbons, such as isobutene and isopentane. (Refer to U.S. Pat. No. 3,615,972)
Such heat-expandable microcapsules are processed into lightweight hollow particulates (heat-expanded microspheres) by heating and expanding. The hollow particulates are employed in various uses, for example, lightening various materials.
The heat-expandable microcapsules are usually produced in suspension polymerization in which an oily mixture comprising a polymerizable component, a blowing agent, and a polymerization initiator is dispersed in an aqueous dispersing medium, and the polymerizable component in the oily mixture are polymerized.
At the polymerization step, all of thermoplastic resin converted from the polymerizable component should preferably constitute the shell of the microcapsules. An example of such heat-expandable microcapsules is shown in FIG. 1. In an actual process, however, all of thermoplastic resin converted from the polymerizable component does not necessarily constitute the shell of heat-expandable microcapsules, and it is often observed that some of the thermoplastic resin is formed into particulates to exist inside the shell of resultant heat-expandable microcapsules. Specifically such phenomenon is noticeable when microcapsules have a comparatively large particle size (for example, a particle size of 25 micrometers or more) or high expansion-initiating temperature (for example, an expansion-initiating temperature not lower than 110 deg.C.). An example of heat-expandable microcapsules containing resin particulates inside their shell (multinucleus microspheres mentioned below) is shown in FIG. 2.
When a lot of such resin particulates are generated, the thickness of the shell (hereinafter sometimes referred to as shell thickness or membrane thickness) of resultant microcapsules becomes thinner than its theoretical value. Such shell prevents the heat-expandable microcapsules from expanding sufficiently to a desirable degree in heating and expanding process, and makes the resultant hollow microspheres have a true specific gravity greater than an expected level, or causes low heat-expandability, such as low expansion ratio, of the heat-expandable microcapsule. Thus the generation of the resin particulates must be controlled, though the control means has not been specifically studied yet.